Compound stage MEM actuator suspended for multidimensional motion

ABSTRACT

A microelectromechanical compound stage microactuator assembly capable of motion along x, y, and z axes for positioning and scanning integrated electromechanical sensors and actuators is fabricated from submicron suspended single crystal silicon beams. The microactuator incorporates an interconnect system for mechanically supporting a central stage and for providing electrical connections to componants of the microactuator and to devices carried thereby. The microactuator is fabricated using a modified single crystal reactive etching and metallization process which incorporates an isolation process utilizing thermal oxidation of selected regions of the device to provide insulating segments which define conductive paths from external circuitry to the actuator components and to microelectronic devices such as gated field emitters carried by the actuator.

This invention was made with government support under Contract No.DABT-63-92-0019 awarded by the Advanced Research Projects Agency (ARPA).The government has certain rights in the invention.

This is a divisional of application Ser. No. 08/443,331 filed May 17,1995, now U.S. Pat. No. 5,506,175, which is a division of Ser. No.08/069,725, filed Jun. 1, 1993, now U.S. Pat. No. 5,536,988.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to compound stagemicroelectromechanical (MEM) devices for multidimensional motion, andmore particularly to MEM sensors and microactuators having multiplesuspended stages interconnected by way of multiple beams to permitselective activation of the stages for controlled motion in threedimensions, and to such devices incorporating microelectroniccomponents, such as gated field emitters. The invention further relatesto methods for fabricating compound micron-dimensioned devices forselective control thereof and for fabricating gated field emittersintegrally therewith.

Microeletromechanical devices have been developed for a variety ofpurposes, and are exemplified by U.S. Pat. No. 5,072,288; U.S. Pat. No.5,179,499; U.S. Pat. No. 5,198,390; and U.S. Pat. No. 5,199,917; allissued to MacDonald et al and assigned to the assignee of the presentapplication, as well as by the article entitled "A Process forSubmicron, Silicon Electromechanical Structures", Zhang et al, J.Micromech. Microeng., Volume 2, No. 1, March 1992, pages 31-38. Variousprocesses and techniques have been developed for production of suchdevices, as described in the Zhang et al publication, and these devicesand processes have created a new technology for micron and submicrondevices.

Fundamental to the application of MEM techniques to micromachinedsystems and instruments, however, is the provision of microactuatorscapable of supporting various microelectronic devices and components toenable them to be moved or held in place selectively and with greatprecision. Particularly needed is a way to position microactuators aboutthree linear (x, y, z) and three angular (roll, pitch, yaw) directions.Such six-way positioning must have a wide range of travel, must operatewith speed and high resolution, must be repeatable, and must have thecapacity to move a significant load, as well as to provide accurateposition readouts, so as to provide accurate positioning as well asscanning operations. Furthermore, to enable such actuators to carrymicroelectronic devices, a mechanism must also be provided forelectrically connecting the microelectronics to exterior circuitry.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide acompound stage MEM device capable of supporting microelectroniccomponents and further capable of providing electrical connectionsbetween such components and exterior circuitry.

A further object of the present invention is to provide a microactuatordevice capable of motion in six directions and of carrying andpositioning electronic components.

A still further object of the invention is to provide amicroelectromechanical structure having compound stages, electricalinterconnections between the stages and components carried by thestages, and to provide a process for fabricating the same.

Another object of the invention is to provide mechanically connected,electrically isolated silicon suspended beam structures for positioningand scanning integrated micromachined optical and electronic devices.

Still another object of the invention is to provide a compound stagemicroactuator fabricated from multiple suspended silicon beamsmechanically interconnected and incorporating electrically isolatingsegments to provide multiple paths for addressing integrated emittertips, gate electrodes, actuators, and sensors.

A further object of the invention is to provide a method of fabricatinga microactuator incorporating multiple interconnected suspended siliconbeams having integrated metal electrodes for electrical actuators andincorporating integral oxide insulators, and to a process forfabricating gated field emitters on such suspended beams.

Briefly, the present invention is directed to a microelectromechanicalcompound stage microactuator and to an interconnect system formechanically supporting a central stage and for providing electricalconnection paths to devices carried by the actuator. The microactuatoris fabricated using a modified and extended single crystal reactiveetching and metallization process (SCREAM) of the type described in U.S.Pat. No. 5,198,390, but including an isolation process utilizing thermaloxidation of selected regions of the MEM structure interconnect systemto provide defined conductive paths from external circuitry to variousselected actuator components and to microelectronic devices carried bythe actuator.

The central stage is mounted in a frame support assembly which includesfirst, second, and third interconnected, coplanar, concentric framestages. The central stage is fixed to, and moves with, the first framestage which, in turn, is mounted within and is movable with respect tothe second frame stage. This mounting is by means of second stage x-axisand y-axis connectors which interconnect the frames and provide therelative motion, for example along an x-axis lying in the plane of themicroactuator structure and passing through the center of the centralstage. The y-axis connectors which secure the first frame stage withinthe second frame are a plurality of spaced elongated springs whichextend parallel to the y-axis of the assembly and are flexible in the xdirection. The x-axis connectors are drivers, such as comb-shapedcapacitors, for producing motion of the first frame stage, and itsconnected central stage, along the x-axis with respect to the secondstage frame, when the drivers are activated. This first frame stage maybe referred to as the x stage.

The second frame stage is mounted within a third frame stage by thirdstage x-axis and y-axis connectors which provide relative motion of thesecond frame with respect to the third frame, for example along they-axis. The third frame stage x-axis connectors are a plurality ofelongated, spaced springs parallel to the x-axis and flexible in the ydirection. The y-axis connectors are drivers for producing y-axis motionof the first and second frame stages and the central stage with respectto the third frame stage, when the drivers are activated. This secondframe stage may be referred to as the y stage.

The third frame stage is suspended within a substrate cavity by aplurality of spaced, elongated x-axis and y-axis suspension beams, theinner ends of which are connected to the third frame, and the outer endsof which are connected to silicon-on-insulator (SOI) structures on asurrounding substrate. These SOI structures clamp the assembly to thesubstrate, while electrically insulating it from the substrate, andpermit electrical connections from the beams to corresponding contactpads on the substrate. The suspension beams are used as mechanicalcantilevered supports and as electrical interconnects to address theactuator drivers, sensor and feedback electrodes, and integratedcomponents, such as gated field emitters, carried by the actuator. Thesebeams are flexible vertically, the vertical flexibility allowing motionof the third frame in the z direction, perpendicular to the x-y plane ofmotion of the first and second frames.

Each of the support frames is fabricated from multiple single crystalsilicon (SCS) beams fabricated from the silicon substrate, and aresubmicron scale structures nominally 300-800 nm wide and 2-4 micrometersthick, after completion of the fabrication process, including a silicondioxide layer conformally covering them. Isolating segments having thincross-sections are provided in the SCS beams so that during thefabrication process the oxidation step which produces the silicondioxide coating also produces complete oxidation of the beams in suchsegments to thereby provide spaced electrical insulator segments atselected locations along the beams. The multiple beams of each of theframes are interconnected by bridges which may either be conductive toprovide electrical connections between adjacent beams, or may befabricated with thin cross sections in selected locations for oxidationto provide insulating support structures between adjacent beams, asrequired for the interconnect system. By carefully selecting thelocation of such insulator segments and bridges, conductive interconnectpaths through the multiple frame beams are provided.

The relative displacements of the first and second stages and the secondand third stages produce corresponding x and y displacements of thecentral stage, and this motion can be driven, sensed, and controlled bydrivers such as the comb-shaped capacitive electrodes integrated in thestages. An electrical potential difference can be applied between theoutermost or third frame stage and electrodes on the floor of the cavityto provide motion in the z direction, so the third frame may be referredto as the z stage. Rotational motion about the z axis of the actuator(yaw) can be controlled or restricted by the provision of electricalpotential differences between corners of the outermost frame stage andthe surrounding walls of the silicon substrate. Similarly, pivotalmotion about the x and y axes (pitch and roll) can be provided orcontrolled by the application of potential differences between segmentsof the frame stages and selected electrodes surrounding the frame stagesor on the floor of the cavity. Accordingly, multidimensional motion ofthe actuator can be precisely controlled with applied drive, sense, andfeedback signals.

The microactuator of the present invention offers capabilities which areuseful for high resolution, 3-dimensional accelerometers, sensors, andpositioning and scanning instruments. The actuator device has excellentresistance to thermal interference because the mechanical and physicalproperties of the single crystal silicon material. Thestage-within-a-stage scheme provides simultaneous x, y, and z motions,while the multiple beam structure of the frames resists stage twist ortorsion. The displacement of the center stage is highly precise becausethe drive, sense, and feedback electrodes used to control it areintegrated into the suspended and movable composite stage and arecontrolled by selected electrical potentials. The bridges whichmechanically interconnect the circumferential beams of the framesprovide electrical connections, but can be oxidized to provideelectrical insulation while maintaining mechanical strength, and themultiple interconnections to the surrounding substrate allow selectiveaddressing of the various components integrated into the assembly.

In a preferred form of the invention integrated microdynamic emissioncathodes are provided on the central stage of the actuator and aremechanically positioned and scanned by the compound x, y, z stages. Theinvention also includes a process for fabricating such emitters which isfully integrated with the extended SCREAM process for fabricating thesingle crystal beams and insulators for the microactuator, as well aswith the process for making the silicon-on-insulator connections betweenthe actuator and the substrate. The process enables emitters to beformed on released silicon beams, with the emitters having tips andgates fully self-aligned with each other and with the beams to permitfabrication of large, dense arrays of such emitters.

As indicated above, the actuator assembly of the present invention isfabricated utilizing a silicon single crystal reactive etchingmetallization process integrated with beam-to-beam isolation andbeam-to-substrate isolation processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, and additional objects, features, and advantages of thepresent invention will become apparent to those of skill in the art froma consideration of the following detailed description of a preferredembodiment, taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagrammatic top plan view of a compound stage microactuatorconstructed in accordance with the present invention;

FIG. 2 is an enlarged partial view of the central stage and a firstframe stage of the assembly of FIG. 1;

FIG. 3 is an enlarged partial view of the microactuator assembly of FIG.1, showing portions of the central stage and x, y, and z support stageassemblies;

FIGS. 4(a) through 4(j) illustrate in tabular form a process forfabricating the microactuator of FIG. 1 at representative cross-sectionsA, B, and C thereof;

FIG. 5 is a perspective enlarged view of an emitter for the centralstage of the microactuator;

FIG. 6 is a cross-sectional view of the emitter of FIG. 5.

FIGS. 7(a)-7(j) illustrate in tabular form a process for fabricating afully self-aligned gated field emitter integrated on a silicon beam; and

FIGS. 8(a)-8(e) illustrate in tabular form a process for fabricating afully self-aligned gated field emitter integrated on a released siliconbeam.

DESCRIPTION OF PREFERRED EMBODIMENT

In accordance with one embodiment of the invention, a compound stageactuator 10 is fabricated from a single crystal silicon substrate 12 asa released, cantilevered structure within a cavity 14 formed in thesubstrate. The released structure 10 includes a central stage portion 16connected to a multistage support assembly 18 which holds the centralstage above the floor 20 of the cavity for 6-way motion in x, y, z,roll, pitch, and yaw directions. This central stage portion 16 mayincorporate microelectronic devices such as cathode emitters 22,capacitor plates, sensors or the like, which preferably are fabricatedusing the SCREAM process (to be described) or a process compatible withit, at the same time the remainder of the structure is formed.Alternatively, microelectronic devices or the like can be fabricatedafter completion of the stage and support structures. The multistagesupport assembly 18 provides an interconnect system which connectsexternal signal sources or receivers (not shown), which may beincorporated in substrate 12, to selected components, such as emitters22, carried by the central stage portion 16 as well as to components ofthe support assembly for use in control or sensing operations. Theinterconnect system uses parts of the support assembly itself as theelectrically conductive elements, with suitable insulating segmentsbeing provided to define specific signal paths through the assembly.These insulating segments provide mechanical connections between variouscomponents of the structure to provide structural integrity whileproviding electrical isolation between adjacent parts to permit complexinterconnection schemes.

The central stage portion 16 (FIG. 2) is supported mechanically within afirst support frame stage 24 which includes a frame comprising areleased beam 26 surrounding the stage 16 and spaced above the floor 20of the cavity 14 in the substrate 12. The central stage 16 is fixedlysecured within this first, innermost beam, or frame 26 so that they movetogether. In a preferred form of the invention, the frame 26 isrectangular, including opposite sides 28, 30, and 32, 34 surrounding thecentral stage portion 16, and is coplanar therewith.

The first support frame stage 24 includes a plurality of interconnectbeams 36-38 and 40-42 extending inwardly from the frame sides 28 and 30,respectively, to secure the central stage 16 in place. The beam 26extends circumferentially around the central stage and provides a rigidsupport frame which will not twist under applied forces, to therebypermit precision positioning of the central stage. The beam 26 formingthe frame preferably is fabricated with a high aspect ratio forstiffness in the vertical direction. Although illustrated as a singlebeam, it will be understood that frame 24 may incorporate pluralparallel circumferential beams, with adjacent beams being interconnectedby bridges to further increase frame rigidity and to provide electricalconnection between adjacent circumferential beams in the mannerdescribed below for the outer frames.

This first support frame stage 24 is mounted within a second, orintermediate support frame stage 50 which preferably is constructed ofplural, parallel released beams such as beams 52-54 (FIG. 3),circumferentially surrounding and spaced from the first frame 24 andcoplanar therewith. The second frame stage 50 is preferably rectangular,with its edges 56, 58 and 60, 62 being parallel to and spaced fromcorresponding edges 28, 30 and 32, 34 of the first frame beam 26. Thefirst frame 26 is mounted to the second frame by x-axis and y-axisconnectors 64 and 66, respectively, which support the first frame formotion along one axis with respect to the second frame. In theillustrated example, the first frame stage 24 is supported for motionalong an x-axis 68 passing through the center of the central stage 16.

Furthermore, in the illustrated embodiment the y-axis connectors 66 ofthe second frame stage 50 are in the form of elongated springs 70, 72and 74, 76, which are released beams extending parallel to the y-axis 78of the device and connected at their opposite ends to correspondingedges of the first and second frames 24 and 50. These y-axis springbeams are laterally flexible to allow the frame 24 to move along thex-axis with respect to frame 50. Preferably these spring mounting beamshave high aspect ratios so that they are flexible in the x direction,but are substantially inflexible in the z direction, which isperpendicular to the plane of the device, so that motion of stage frame24 with respect to the stage frame 50 is constrained to the x-y plane ofthe first frame 24 along the x-axis. Motion of the central stage 16follows that of stage 24.

Motion of stage 24 with respect to stage 50 is produced by, and may besensed by, comb-type drive capacitors 80 and 82 having first plates 84and 86 extending outwardly from opposite sides 32 and 34, respectively,of stage 24 and interleaved with second plates 88 and 90 extendinginwardly from corresponding opposed edges 60 and 62 of the frame 50. Theplates 84 and 88, and plates 86 and 90 are interleaved so that theapplication of potentials across the respective drive capacitor 80 and82 produces corresponding controlled motion in the x direction, or inthe alternative, potentials across these plates can be measured todetermine the location of the inner frame 24 with respect to frame 50.

Frame stage 50 is, in turn, mounted to an outermost frame stage 100which is similar in structure to that of frame 50 and thus is comprisedof plural, parallel, mechanically interconnected, released beams 102,103, and 104 extending circumferentially around and spaced from theinner frame 50. The outer frame stage 100 thus is concentric with theinner frames 24 and 50 and with the central stage 16, and preferably isrectangular, with sides 106, 108 and 110, 112 being parallel tocorresponding sides 56, 58 and 60, 62 of the inner frame 50. Because ofits interconnected multiple beam structure, the frame 100 issufficiently rigid to resist twisting. The plural beams provide multipleelectrical paths for use in interconnecting the drive capacitors and thestage microelectronics to external circuitry, as will be described.

The inner frame 50 is mounted to the outer frame 100 by means of plural,coplanar x-axis spring beam connectors 114-117 and 120-123 which areflexible in the y direction to permit relative y-axis motion between theinner and outer frames. Motion along the y-axis is produced by comb-typecapacitors 126 and 128 connected between corresponding sides 56, 106 and58, 108 of the inner and outer frames 50 and 100, respectively. Eachdrive capacitor includes two sets of interleaved plates, one set (130 or132) being connected to a corresponding inner frame edge (56 or 58) andthe other set (134 or 136) being connected to a corresponding outerframe edge (106 or 108) so that the application of a potential acrossthe capacitors produces motion of frame 50 with respect to the outerframe 100 along the y-axis.

Frame 100 is suspended within cavity 14 formed in substrate 12 by meansof cantilevered, released, coplanar suspension beams 140-142 and 144-146extending between ends 106 and 108 of frame 100 and corresponding edges150 and 152 of the surrounding substrate. Similarly, suspension beams154-157 and beams 160-163 extend between frame edges 110 and 112 andcorresponding edges 164 and 165 of the substrate 12. These suspensionbeams hold the frame 100 in the x-y plane, securing it against motionalong either the x or y axis. However, the outer frame 100 is capable ofmotion in the Z direction, perpendicular to the plane of the frame,toward and away from the substrate cavity floor 20. This motion may beunder the control of a potential applied between a conductive layer onthe floor 20 of cavity 14 and corresponding conductive beams on themovable frame stage 100, since the suspension beams 154-157 and 160-163can flex in the vertical direction.

Furthermore, all of the suspension beams connecting the frame 100 to thesurrounding substrate can flex vertically to allow rotational motion ofthe outer frame 100 with respect to the substrate and around the z axis,which is perpendicular to the xy plane. This allows the entire actuatorassembly 10 to be moved in clockwise or counterclockwise directions inthe x-y plane. Such rotational, or yaw motion, can be produced by meansof potentials applied across conductive beam segments at the corners offrame 100 and adjacent capacitive plates (indicated by dotted lines 166)on adjacent walls of cavity 14. Furthermore, by providing a conductivelayer on the cavity floor, and dividing the floor conductive layer intosegments; for example, into four quadrants 168a, b, c, and d indicatedby dotted lines, and by application of potentials between selectedquadrants and parts of the movable frame 100, controlled pivotal motionabout either the x or the y axis can be produced for pitch and rollmotion of the central stage 16.

The suspension beams are secured at their outer ends to correspondingconnectors fabricated by a silicon on insulation (SOI) process wherebyelectrical connections can be made to the beams from external circuitryby way of contact pads and suitable conductors formed on the substrate.As illustrated in FIG. 1, suspension beams 140-142 are connected toconnector pads 170-172, respectively and beams 144-146 are connected topads 174-176, respectively, through SOI connections, with each of thepads then being electrically connected by way of correspondingconductors 178-180 and 181-183 to control or sensing circuits (notshown) which may be incorporated in the substrate 12 or may be externalthereto. Similarly, the outer ends of beams 154-157 and 160-163 areconnected through corresponding SOI connections, pads 184-187 and190-193 and conductors 194-197 and 200-203 to respective control orsensing circuits.

As illustrated diagrammatically in FIG. 1 and in greater detail in theenlarged view of FIG. 3, the circumferential beams 52-54 which make upframe 50 are interconnected by way of mechanically supportive bridgeconnectors such as those illustrated at 210. These bridge connectors arefabricated integrally with the circumferential beams 52, 53, and 54 andare mechanically strong to provide a rigid frame structure, as well asto provide electrical connection between the circumferential beams. Insimilar manner, the circumferential beams 102-104 which make up frame100 are mechanically and electrically interconnected by bridgeconnectors such as those illustrated at 212, to make a rigid framestructure.

The inner ends of suspension beams 140-142, 144-146, 154-157, and160-163 are mechanically connected at spaced locations to the outerframe 100, and preferably are integral therewith, to suspend the frame,and thus the entire actuator assembly 10, within cavity 14 and spacedabove floor 20. These suspension beams provide electrical connectionsbetween the connector pads on the substrate and the frame 100, and areelectrically connected to the outer drive plates 134, 136 of capacitors126 and 128 or through selected inner conductor beams to the innerframes and to the central stage. Although these suspension beams areillustrated as being single beams, it will be apparent that they can befabricated as multiple parallel beams interconnected by bridges spacedto provide the desired rigidity or flexibility.

The plural circumferential beams making up each of the inner and outerframes 50 and 100 and the interconnecting bridges 210 and 212 whichmechanically strengthen the frames, are normally electricallyconductive, and thus can be used to conduct electrical signals fromexterior circuitry to selected components of the actuator assembly.Electrically insulating segments such as segments 214 and 216 in frames50 and 100, respectively, selectively divide the circumferential beamsinto plural isolated electrical segments. Further, selected bridges suchas those illustrated at 218 and 220 are also electrically insulating,and cooperate with insulating segments in the circumferential beams todefine isolated electrical paths by which each of the outer conductorscan be connected to a preselected component of the assembly. Forexample, outer suspension beam 160 can be connected through selectedsegments 222-224 and bridges 225 and 226 of frame 100, support beam 120and segment 227 of beam 54 of frame 50 for electrical connection to thedrive capacitor plates 130 on frame 50. Similarly, outer suspension beam142 can be connected through selected segments of beams 102-104 makingup the outer frame 100 and through selected bridges to capacitor plates134 interleaved with the aforementioned plates 130. Another suspensionbeam 161 can be connected to the capacitor plates 86 on frame 24, asthrough selected segments of frames 50 and 100 and correspondingconnector beams 121 and 72, and so on. By careful design and positioningof insulating segments, electrical connections can be made as requiredto activate the compound stage actuator device of the invention to movethe central stage 16 in three directions. Similar connections tocapacitive plates on the floor and walls of the cavity and on frame 100can provide three directions of rotational motion.

The central stage may support a variety of components, including varioussensors or, in one embodiment, an array 230 of field emission cathodes22 for use in vacuum microelectronic devices and applications. Suchemitters are electrically connected to external circuitry through theconnector beams and frame segments as described above. These beams andframe segments are resistive, and thus function as series emitterresistors to limit sharp rises in current not only to protect theemitter cathodes, but to provide improved uniformity of emission where aplurality of such cathodes are provided. In one embodiment of theinvention, the central stage 16 was capable of scanning a 20×20micrometer area in the x-y plane and was capable of moving up and down±500 nm. Further, it was capable of operating at scan rates of greaterthan 20 kHz.

A process for fabricating the single crystal silicon actuator 10described above is outlined in diagrammatic tabular form in FIGS.4(a)-4(j) which illustrate the effect of the process steps at differentcross sectional regions of the actuator structure. Cross section Aillustrates the various electrically conductive beams or beam segments,such as the circumferential beams 52-54 and 102-104, as well as thesuspension beams securing the frame 10b within cavity 14, the connectorbeams between frames, the bridges, the various capacitor plates, etc.This cross section may be taken, for example, at A in FIG. 3 on beam 54.In similar manner, cross section B, also found in FIG. 3 on beam 54,illustrates the cross sectional region of beam segments which are to becompletely oxidized during the process to form insulating segments.Cross section C at connector pad 172 in FIG. 1, is an illustration ofthe process for fabricating a silicon on insulator (SOI) connectionbetween the suspension beams and connector pads formed on the substrate.

The starting substrate 12 is an arsenic-doped 0.005 ohm-cm, n-type,(100) silicon wafer. A layer 240 of silicon dioxide, preferably 500 nmthick, is thermally grown on this silicon substrate in a pyrogenic steamoxidation at 1100° C., and is used as an etch mask, identified in FIG.4(a) as mask 1. The pattern for producing the released single crystalsilicon structure is created using a photolithographic techniques. Toobtain nanometer-scale feature sizes by this process, acontrast-enhancement material is used with-KTI-OCG 895i 5 cs photoresistsuch as the photoresist layer 242 for patterning. This photoresistmaterial is spun on the top surface of the silicon dioxide layer 240 at6 krpm to a thickness of 0.5 micrometers. The desired pattern for theactuator assembly 10 is then used for a photolithographic exposure ofthe photoresist material, defining the desired width and length of eachof the components of the actuator. As illustrated in FIG. 4(a), thepattern line width of the structural beams is about 500 nm, asillustrated at cross section A, while the pattern line width for thosebeam segments which are to provide oxide isolating segments is about 200nm, as illustrated at cross section B.

The photoresist pattern 242 is transferred to the silicon dioxide layer240 in a CHF₃ /O₂ plasma etch at flow rates of 30 sccm/0.5 sccm at achamber pressure of 30 mTor and a DC bias of--440 volts in a customparallel plate RIE system. The etch rate of the silicon dioxide is 230nm per minute. Thereafter, the photoresist is stripped by an O₂ plasmaetch. After the photoresist is stripped, the resulting silicon dioxidepattern is transferred to the silicon substrate, in the mannerillustrated in FIG. 4(b), using a Cl₂ -RIE. The Cl₂ etching removes anyorganic residue and native oxide on the silicon surface, and thereaftera silicon etch is performed in a Cl₂ /BCl₃ plasma etch at flow rates of49 sccm/7 sccm at a chamber pressure of 20 mTorr and a DC bias of -400volts. The depth of the silicon etch illustrated in FIG. 4(b) is 3micrometers, leaving mesas, or islands, 244 which are to become thevarious beams and bridges of the actuator. A 310 nm thick silicondioxide layer 240 is left on top of the etched silicon lines 244.

Following the silicon etch, a sidewall silicon dioxide layer 246 isthermally grown to a thickness of 250 nm in a steam O₂ ambient at 1000°C., as illustrated in FIG. 4(c). The width of the mesas 244 are reduceddue to the consumption of silicon during the oxidation step, so that thewider mesas are reduced, as indicated at 248 in cross sections A and C,and the narrower line segments (initially 200 nm wide) are fullyoxidized, as illustrated at 250 in FIG. 4(c), cross section B. As notedabove, the narrower line segments can be placed at any desired locationwithin the actuator to provide the desired oxidized segments forelectrical insulation, these segments still being sufficiently wide toprovide the required mechanical connection and strength for thestructure. The oxidized segment 250 may correspond, for example, to thesegments 214, 216, 218, and 220 illustrated in FIG. 3.

As also illustrated in FIG. 4(c), a 100 nm layer of LPCVD siliconnitride 252 is conformally deposited on the silicon dioxide layer 246.This silicon nitride (Si₃ N₄) serves as an oxidation mask to protect thebeam structure during subsequent oxidation steps which occur during theformation of the SOI structures.

In some cases, the demands of the structural design may require that thebeam width be greater than that which will permit complete oxidationduring the steps illustrated at FIG. 4(c), yet complete oxidation isrequired for isolation at that location. In such a case, the isolationprocess described above is followed by a selective oxidation step beforethe SOI structure is formed. In this case, a dielectric stack ofnitride/oxide (100 nm/100 nm) is deposited by LPCVD and PECVD. Thenitride on the top and upper part of the sidewalls of the siliconsegments which are to be oxidized is selectively stripped, using anoxide etch mask, by hot (160° C.) phosphoric acid solution to provide anoxidation window opening which allows the silicon segments to be fullyoxidized, while the structural base-silicon is not oxidized. This isfollowed by a second thermal oxidation to completely oxidize theselected segments, and a conformal layer of silicon nitride is providedto cover the window.

Thereafter, as illustrated in FIG. 4(d) a photoresist layer 254 is spunon the silicon nitride layer 252, for example, at 2.50 krpm in 60seconds to a thickness of 3.5 micrometers, as illustrated in FIG. 4(d),cross sections A and B. A second mask is then used to pattern thedesired SOI structures in the photoresist material, this pattern beinggenerally indicated at 256 in FIG. 4(d), cross section C, taken at theedge 150 of substrate 12, as illustrated in FIG. 1. This SOI structureis used to mechanically connect a beam to the substrate and to providean electrical connection between the beam (such as beam 142 in FIG. 1)and its corresponding conductive pad 172, so that a connection can bemade to circuitry on the substrate or at some other desired location.The pattern 256 of the SOI structures defines each of the connectors atthe outer ends of suspension beams 140-142, 144-146, 154-157, and160-163.

After the pattern has been defined in the photoresist, as through aphotolithographic exposure, the nitride/oxide exposed by the pattern isvertically etched back with CHF₃ /O₂ plasma etch, again as illustratedat cross section C in FIG. 4(d). The vertical etch removes thedielectric stack on horizontal surfaces, for example at the base of themesa 244 (cross section C) while leaving at least some of the silicondioxide mask 240 on the top of mesa 244 and the silicon dioxide 246 onits sidewalls, while also leaving the silicon nitride 252 on thesidewalls.

The next step in the process is an isotropic silicon recess etch,illustrated at FIG. 4(e), which is performed using a fluorinated silicon(SF₆) RIE. This recess etch, illustrated at 258 (in FIG. 4(e), crosssection C) removes silicon from the top layer of substrate 12 and etchesunder the edges of the sidewall nitrate 252 to produce the undercutregion 258. This recess etch enhances the lateral oxidation at the baseof the silicon line 244 during the subsequent oxidation step byincreasing the transport of oxidizing species underneath the singlecrystal silicon line. This recess etch has no effect at cross sections Aand B.

The photoresist layer 254 which had been used to protect the beamstructures is then stripped by an O₂ plasma etch, as illustrated in FIG.4(f) and an 1100° C. pyrogenic steam oxidation is performed to grow a700 nm thick field oxide 260 which laterally undercuts the singlecrystal silicon line 244 at cross section C, thereby producing theisolated beam segment 262 (FIG. 4(f) at cross section C), which beamsection may be a part, for example, of suspension beam 142 in FIG. 1.The single crystal silicon lines 248 and 250 at cross sections A and Bare protected from the oxidation step by the silicon nitride layer 252.

Following the oxidation step of FIG. 4(f), metal-contact windows areopened on the top of the single crystal silicon lines, where required,and over corresponding contact pads previously formed in the surroundingsilicon wafer 12. Such contact pads in the wafer may have beenfabricated during the formation of electrical circuits on the surface ofthe wafer, the pads being covered by a layer of oxide prior to formationof the actuator 10. In step 4(g), such pads may be exposed by the windowmask (Mask 3) formed by conventional photolithography in a photoresistlayer 264. For such metal-contact window patterning, the photoresist ontop of the beam 262 is exposed through its complete thickness, while theresist material to the sides of the beam region is sufficiently thick toprevent exposure completely through the photoresist. Therefore, theexposed photoresist is completely developed away on top of beam 262, asat region 266 (FIG. 4(g), cross section C) so that the silicon dioxidelayers 240 and 260 on top of the silicon beam 262 are etched using CHF₃/O₂ plasma etch to reveal the top surface 268 of beam 262. In similarmanner, the contact pad for this beam may also be opened, after whichthe photoresist 264 is stripped away using an O₂ plasma etch.Thereafter, the silicon nitride layer 252 is stripped, using hotphosphoric acid at 160° C., as illustrated in FIG. 4(h).

Metallization of the metal contact window is completed by an aluminumlift off using a tri-layer resist process. A 300 nm layer of aluminum isapplied, using mask 4 (FIG. 4(i)), the aluminum layer 270 conformallycovering the SOI structure 262. A DC magnitron sputter depositionprocess may be used to provide the layer 270 (cross section C). A metaletch step is then used to remove excess metal.

The silicon dioxide 246 on the horizontal surface of substrate 12 at thebase of the lines 248 and 250 is then removed by a second etch backusing CHF₃ /O₂ plasma etch. The thicker field silicon dioxide remains onthe top surface of the lines 248 and on the sidewalls thereof, while thealuminum layer 270 protects the oxide layer 260 beneath the beam 262(cross section C). Finally, the lines 248 and 250 are completelyreleased from the silicon substrate 12 using an SF₆ RIE process in thesame etch conditions as were used in the silicon recess etch, therebyproviding released beam 248 and released beam 250, illustrated in FIG.4(j) at cross sections A and B. It is noted that the beam 248 isprotected by the silicon dioxide layer 246 during this last releasestep. The beam 248 thus corresponds to the silicon beams utilized in theactuator assembly 10, as discussed above. The silicon dioxide layer 260for the SOI structure is not etched, leaving the SOI structure clampingthe ends of the beams to the substrate.

As illustrated in FIGS. 5 and 6, released silicon beams such as thebeams 248 illustrated at cross section A in FIG. 4(j) can incorporateone or more field emission cathodes, such as the cathode 22 illustratedin FIG. 2. A single such cathode is illustrated in FIG. 5 at 280, thiscathode being formed on a beam 282 which may, for example, be one of thebeams forming a part of central stage 16 and illustrated in FIG. 2.

FIGS. 7(a) to 7(j) illustrate a process for building a gated fieldemitter on a high aspect ratio silicon structure with submicron lateralfeature size. The silicon base for the emitter can be released orunreleased; in the illustrated embodiment of FIGS. (7a)-7(j) the base isunreleased, but the process can be combined with that described abovefor actuator 10 to provide emitters on released beams, as will bedescribed below.

To form a tip for an emitter, a 350 nm thick layer 284 of P(MMA-MAA)(9%) copolymer resist on a substrate 286 and dielectric layer 288 and289 is electron-beam exposed in a Cambridge Instruments EBMF-10.5/CSsystem at 20 kV beam energy and 1.5 nA beam current on a 1.6384 mmexposure field (see FIG. 7(a)). The tip is defined by a 500×500 nm²island 290 surrounded by a 150 nm wide trench 292. The pattern in thedeveloped electron-beam resist is transferred to underlying dielectricfilms of plasma enhanced chemical vapor disposition (PECVD) oxide 288(100 nm) and low pressure chemical vapor deposition (LPCVD) nitride 289(100 nm) using CHF₃ /O₂ RIE, and then to the substrate-silicon 286 usingSF₆ RIE (see FIG. 7(b)). The silicon etch produces a 1 μm deep trench294 with 200 nm lateral undercut under the oxide film 288, which is usedas an etch mask, and produces a central island 295.

A 100 nm thick oxide layer 296 thermally grows on the exposed silicon(see FIG. 7(c)). The nitride film 289 that remains from the previoussilicon etch step serves as an oxidation mask. This oxidation stepprovides an oxide layer which reduces the size of island 295 as well asthe sidewalls 300 and bottom 302 of the silicon trench to producesilicon tip 304, which is sharpened and, thereby, has a thicker oxidecovering on the tip apex due to the isotropic reduction of thedimensions by consumption of some of the surface silicon.

After the oxidation step, a planarization process is used to completelyfill the silicon trench 294 (see FIG. 7(d)) with oxide 306. Theplanarization process is performed with alternative deposition andvertical etchback of PECVD oxide and planarizing material (KTI-OCGphotoresist), and is finished by removing the dielectric stack,including nitride 289, covering over the substrate-silicon. The oxide306 in the trench and covering over the silicon tip is used for asubsequent silicon etch mask to redefine the silicon emitterbase-structures.

A silicon structural beam having a width of 800 nm to support tip 304 isrequired by the 300 nm trench patterning and the tip etch and oxidationdescribed above. To produce such a beam, silicon 286 is verticallyetched back past the level of the trench bottom 302, and then down 3 μmto form the beam structure 310 by a Cl₂ /BCl₃ RIE etch (see FIG. 7(e)).

Following the silicon etch, the oxide etch mask 306 is stripped offusing buffered hydrofluoric acid solution. A 250 nm thick oxide film 312is provided to conformally cover beam 310 and tip 304 by depositingPECVD oxide (see FIG. 7(f)). The following gate-electrode metallizationis performed on the oxide film with a conformal, rather than a level,coating.

A sputtered Ti₀.1 W₀.9 film 314 is deposited on the oxide film (see FIG.7(g)). A 3.5 μm thick planarizing material 316 (KTI-OCG 895i 50 csphotoresist) is spun on the Ti₀.1 W₀.9 film, the thickness of thephotoresist corresponding to the 3 μm height of the structure 310, 304(see FIG. 7(h)). The photoresist directly above the tip apex is thinnerthan over the remainder of the surface. An O₂ plasma etchback exposesthe Ti₀.1 W₀.9 layer 314 above the apex, at 318. The exposed Ti₀.1 W₀.9is then etched using an SF₆ RIE, most of the Ti₀.1 W₀.9 lying underneaththe photoresist being protected by the photoresist (see FIG. 7(i)), asindicated at 320. This etch does not affect the oxide. The oxide 312covering the tip is then removed using buffered hydrofluoric acidsolution. The remaining photoresist 316 is stripped off (see FIG. 7(j)),leaving the metal layer 314 surrounding the tip 304 and providing a gateelectrode aperture 322 for the tip. The electrode preferably has a gateaperture diameter of about 500 nm, but the diameter of aperture 322 canrange from 200 to 800 nm. The aperture diameter is determined, in part,by the thickness of the conformal oxide film 312.

Although the emitter fabrication process described above can be usedseparately, it can also be fully integrated into themicroelectromechanical actuator 10 of the present invention, to provide,for example, emitters on submicrometer wide beams in the center stageportion 16.

The integrated process starts with the arsenic-doped, 0.005 Ω cm,n-type, (100) silicon substrate 12 (or 286, as shown in FIG. 7(a)) onwhich there are 100 nm thick films of nitride 289 and oxide 288deposited. The first stage in forming the silicon structures is toproduce the 1 μm height silicon tips by producing 1 μm deep trenches294, as shown in FIG. 7(b), as by electron-beam lithography andoxide/nitride etch and silicon etch. The trench widths range from 150 to250 nm. In addition, 150 nm wide trenches can also be provided wheredesired to form 300 nm wide, suspended oxide segments (such as segment250 in FIGS. 5 and 6) to electrically isolate suspended siliconsegments. A 100 nm film 296 of thermal oxide covers the tips and thebottom and sidewalls of the trenches by an oxidation step. The thermaloxidation step further sharpens the tips and provides thicker oxide onthe apexes. A planarization process is performed to fill PECVD oxide inthe trenches. These process steps are referred to in the process outlineshown in FIG. 7(a) to (e).

The second stage of the integrated process is forming high aspect-rationsilicon structures, either with or without emitters. The entire actuatorstructure is defined by the pattern in the oxide filling in the trenchesand covering over the tips in FIG. 7(d) and covering the substrate inFIG. 4(a) (cross sections A, B, and C). A Cl₂ /BCl₃ RIE is performed toetch silicon 3 μm deep using the oxide as an etch mask.

A 100 nm thick film of LPCVD nitride 252 and 312 in FIGS. 4(c) and 7(f),respectively, selectively and conformally covers the beams, includingthe portion of the structures on which the tips are located. Thisnitride film is used to protect the tips during subsequent isolationprocess implementation.

The integrated process continues with isolation and metallizationprocesses as shown in FIGS. 4(d) to 4(j) to produce the composite XYZstages with the integrated electro-mechanical sensors and actuators. Thesuspended silicon structural beams for the emitters are 800 nm wide and3 μm thick, and are coated by conformal oxide film.

After the beam structures are released (see FIG. 4(j), cross-section A,B, and C) and FIG. 8(a) the oxide 246 and 306 on the portion of thestructures with integral silicon tips is etched away and a new conformaloxide 330 is deposited (see FIG. 8(b)). The metallization process of theintegrated emitter gates is performed, as outlined in FIGS. 7(g)-7(j),except that it is performed on a released beam 310. The pattern of thegate electrodes is created using trilayer resist, photolithography andmetal (e.g. sputter Ti₀.1 W₀.9) liftoff. The trilayer resist consists of4 μm photoresist (KTI-OCG), 100 nm PECVD oxide (deposited at 90° C.) and500 nm photoresist. The pattern in the developed photoresist istranferred into the oxide by CHF₃ /O₂ RIE, and then into the basephotoresist 332 by O₂ RIE. The base photoresist is etched by O₂ RIEuntil most of the vertical sidewalls 334 of the released structure 310is exposed (see FIG. 8(c)). After the metal 314 is deposited on theoxide/photoresist stack the excess is "lifted off" by using acetone todissolve the photoresist. Windows for gate aperture openings are definedin redeposited trilayer resist by photolithography and RIE etches ofoxide and base photoresist. The O₂ RIE etch is stopped once the gatemetal above the tip apexes is exposed (see FIG. 8(d)), the metal isetched, and the oxide covering over the silicon tips and the remainingbase photoresist are removed by buffered hydrofluoric acid solution andO₂ RIE, respectively (see FIG. 8(e)), to provide a gate aperturesurrounding a tip on a released beam.

As illustrated in FIGS. 5 and 6, such a released beam 282 mayincorporate a silicon core 310 similar to the beam in FIG. 8(e), whichis released from the floor 20 of substrate 12 in the manner describedabove. Core 310 is covered by a silicon dioxide layer 330 and mayincorporate, for example, one or more oxide segments 250 whichelectrically insulate regions of the beam 282, such as region 340 fromadjacent regions of that beam, such as region 342. An oxide conformallayer 330 is provided on the surface of core 310 in the region of theemitter 280, to insulate the metal gate electrode layer from the siliconcore.

The metal gate electrode 314 contacts the silicon core 310 in regions340 through a window 344 formed in oxide layer 286, while the emittertip 304 is connected by way of beam portion 342 and window 346 to metallayer 270 (see FIG. 4(j), Section C) at the substrate to which the beam282 is connected. In this case, the substrate is the center stage 16 ofthe actuator, through which electrical connections are made throughconductive paths in the actuator beam structure. Similarly, beam section340 can be connected to an SOI section at the opposite side of stage 16(see FIG. 2).

The integrated, mechanically scannable field emission cathode array 230offers unique capabilities for vacuum microelectronic devices andapplications. By selectively connecting the conductive cores 310 of thebeams in the stage 16 to corresponding connector pads on the surroundingsubstrate by way of conductive paths defined in the frames and theirinterconnect structures, the field emitters can be activatedindividually or in groups. As noted above, the conductive beams serve asseries emitter resistors to limit sharp rises in emission current tothereby protect the cathodes and improve uniformity of emission.

Although the present invention has been described in terms of preferredembodiments, it will be apparent that numerous modifications andvariations may be made without departing from the true spirit and scopethereof, as set forth in the following claims.

What is claimed is:
 1. A method for fabricating a microactuator,comprising:forming a central stage containing submicron-scalecomponents; supporting said central stage within first, second, andthird electrically conductive frames for relative motion of said centralstage about an x-axis and a y-axis with respect to said third frame;electrically isolating segments of said frames from adjacent segments;interconnecting said frames through electrically conductive bridgesbetween selected segments of said frames to provide conductiveinterconnect paths from selected segments of said third frame to saidstage components.
 2. The method of claim 1, further including:providingfirst drivers for displacing said first frame with respect to saidsecond frame to displace said central stage about said x-axis; andproviding second drivers for displacing said second frame with respectto said third frame to displace said central stage about said y-axis. 3.The method of claim 2, further including electrically controlling saidfirst and second drivers through said conductive interconnect paths. 4.The method of claim 1, further including supporting said central stagewithin said first, second and third frames for relative motion of saidstage along a z-axis.
 5. The method of claim 4, furtherincluding:displacing said first frame with respect to said second framefor motion about said x-axis; and displacing said second frame withrespect to said third frame for motion about said y-axis.
 6. The methodof claim 4, further including:mounting said third frame to a substrateby way of electrically conductive bridges; and interconnecting circuitelements on said substrate to said stage components through selectedconductive bridges and selected segments of said frames.
 7. The methodof claim 6, further including displacing selected ones of said first,second and third frames with respect to said substrate to move saidcentral stage with respect to said substrate.
 8. The method of claim 7,wherein displacing selected frames includes:providing first drivers fordisplacing said first frame with respect to said second frame; providingsecond drivers for displacing said second frame with respect to saidthird frame; and providing third drivers for displacing said third framewith respect to said substrate.
 9. The method of claim 6, furtherincluding sensing the relative motion of said frames with respect toeach other.
 10. The method of claim 6, further including sensing therelative motion of said first, second and third frames about said x-axisand said y-axis.
 11. The method of claim 10, further including sensingthe relative motion of said third frame with respect to said z-axis. 12.The method of claim 6, further including fabricating said central stage,said first, second and third frames, said interconnecting bridges, andsaid stage components unitarily from said substrate.
 13. The method ofclaim 6, further including fabricating said central stage, said first,second and third frames and said interconnecting bridges of singlecrystal silicon by single crystal reative ion etching and metallizationto provide submicron frame dimensions.
 14. The method of claim 13,further including fabricating said central stage to incorporateintegrated emitter tips.
 15. The method of claim 14, whereinelectrically isolating segments of said frames includes a thermaloxidation step.
 16. A method for fabricating a microactuator,comprising:forming from a substrate a central stage supported by anelectrically conductive first frame; concurrently forming from saidsubstrate second and third electrically conductive frames andinterconnects mechanically and electrically connecting said frames forrelative motion of said central stage with respect to said third frame;and electrically isolating selected segments of each of said frames fromadjacent segments of each frame to provide at least one conductive,electrically isolated interconnect path from said third frame to saidcentral stage through said first and second frames and through saidinterconnects between said frames.
 17. The method of claim 16, furtherincluding mounting said third frame of said microactuator in a substrateby electrically conductive interconnects; andelectricallyinterconnecting elements or said substrate with said central stagethrough selected electrically isolated interconnect paths.